This invention relates to a power brake unit and particularly to a mechanically controlled power brake unit for an automotive vehicle having a master cylinder and a booster utilizing the pressure difference between a vacuum and atmospheric pressure.
From U.S. Pat. No. 3,222,868, a power brake unit is known including a vacuum casing comprising a cup shell and a cover shell, a working piston movable therein including a diaphragm, and a push rod. A master cylinder of a hydraulic brake system is fastened to the front end of the cup shell of the vacuum casing. Bolts are provided on the cover shell which serve to secure the whole unit--power brake unit and master cylinder--to the vehicle.
A similar power brake unit is described in German Pat. No. 1,131,534. In this specification, the master cylinder is provided with an annular flange which is designed as the bottom member for the cylinder of the booster and has the master cylinder bolted thereto. The other end of the booster cylinder is secured to the splash wall, with a cover shell being inserted therebetween.
In arrangements of this type, the parts of the vacuum casing are conventionally made of sheet steel and the master cylinder is made of a grey cast iron casting or a light metal casting. To increase the mechanical load-carrying ability, the vacuum casing parts may be reinforced. When the brake pedal is depressed, the force transmitted from the brake pedal and the booster force are effective in the push rod. The sum of the forces acts on the piston of the master cylinder, the development of a hydraulic pressure in the master cylinder requiring, however, the corresponding forces to be transferred to the booster casing. These forces cause a relatively large extension of the vacuum casing, resulting ultimately in an increased lost travel of the brake pedal.
Further, the known arrangements are unable to comply with the vehicle manufacturers' demand for a lowest possible weight for the brake systems without the safety and operability of the power brake unit being impaired. While it is possible to substitute light metal parts for the sheet steel parts, the resulting increase in the production costs would be substantial. In addition, this measure would not improve the brake pedal's lost travel.